Functions

In this sense, a function is a type of procedure or routine. Some programming languages make a distinction between a function, which returns a value, and a procedure, which performs some operation but does not return a value.

Functions "Encapsulate" a task (they combine many instructions into a single line of code). Most programming languages provide many built in functions that would otherwise require many steps to accomplish, for example computing the square root of a number. In general, we don't care how a function does what it does, only that it "does it"!

When a function is "called" the program "leaves" the current section of code and begins to execute the first line inside the function. Thus the function "flow of control" is:

• The program comes to a line of code containing a "function call".
• The program enters the function (starts at the first line in the function code).
• All instructions inside of the function are executed from top to bottom.
• The program leaves the function and goes back to where it started from.
• Any data computed and RETURNED by the function is used in place of the function in the original line of code

Why are we using functions?

1. They allow us to conceive of our program as a bunch of sub-steps. (Each sub-step can be its own function. When any program seems too hard, just break the overall program into sub-steps!)
2. They allow us to reuse code instead of rewriting it.
3. Functions allow us to keep our variable namespace clean (local variables only "live" as long as the function does). In other words, function_1 can use a variable called i, and function_2 can also use a variable called i and there is no confusion. Each variable i only exists when the computer is executing the given function.
4. Functions allow us to test small parts of our program in isolation from the rest. This is especially true in interpreted languages, such as Matlab, but can be useful in C, Java, ActionScript, etc.